Television system



Feb. 2, 1943.

A. v. BEDFORD TELEVISION SYSTEM Filed Sept. '7, 1940 2 Sheets-Sheet 131mm or fllda I r Bedfo r2;

2. 1 3. v A. v. BEDFORD. 2,309,144

TELEVISION sysmm;

-' Filed Sept. 7,1940

2 Sheets-Shut 2 Fi q. 3.

Patented Feb. 2, 1943 Aida V. Bedford, Collingswood, N. 1.,

assl'gnor to Radio Corporation of America, a corporation of DelawareApplication September 7, 1940, Serial No. 355,797

4 Claims.

This invention relates to television transmitters and more particularlyto transmitters of the type employing cathode ray tubes.

Television signaling systems are adapted to provide images of' highdefinition over a relatively' wide band of signaling frequencies. It iswell known that thermionic amplifying tubes, their associated circuits,and cathode ray transmitter tubes experience a falling off in responseat high frequencies due to the fact that the in-- terelectrodeand'inter'element capacity is of sufficient magnitude to providerelatively low impedance over the upper range of the required frequencyband. One method of overcoming this decreased efficiency in thetelevision system is to designthe output circuit of the cathode raytransmitter tube so that the response of the transmitter tube issubstantially fiat over the useful band of frequencies. This has been accomplished by providing a low value load resistance for the transmittingtube so that the distributed capacity in the transmitter tube would havelittle effect on the response characteristic in the .useful band.Picture signal from the transmitter tube was then fed into I anamplifier designed to have a fiat frequency response whereby the signalappearing at the output of the amplifier was a true representation ofthe conditions of light and shade of the picture. The use of a lowvalued load. resistance in the output circuit of the transmitting tubereduces its output considerably, thus requiring a large amount ofamplification which consequently introduces a large amount of extraneousnoise and thereby deleteriously affecting the resultant transmittedimage.

In order to obtain low noise to signal ratio from a high impedance wideband low output device, such -as a cathode ray transmitter tube, it isadvantageous to operate the device into a load which has high shuntresistance compared to the shunt capacity reactance due to the stray anddistributed capacity of the tube and circuit. At the highest, usefulfrequency the output resistor may well be one hundred or more times ashigh in value as the shunting capacitive reactance.

In the arrangement of my previous electrode capacity.

Patent No. 2,151,072, the loss in response at v high frequency resultingfrom a transmitter tube provided with a relatively high load resistancewas equalized by an inductance connected in series with a in asubsequent amplifier stage.

In order that the effect-of low-valued plate resistor the interelectrodecapacitance may be negligible, the reactance of the inductance at thehighest useful, frequency must be small compared to that of the inter-The value of the load resistance will be only one hundredth of thereactance of the inductance, say 10 ohms, and

hence will be so small that at low frequency,

the signal on the control electrode of the falling amplifier stage willbe so small that the low frequency interference due to unsteady powersupply of the amplifier tubes may be objectionable.

This difficulty is greatly reduced by equalizing for the fallingfrequency characteristic in two or, more amplifier stages, thus allowinggreater low frequency gain in each of the amplifier stages since thehigh frequency gain is limited by the interelectrode capacitance.

Two or more not be made to provide equalization, according to thecorrect law, to compensate for a single I input circuit. This is evidentwhen it is considered that for all except the very low frequencies, theinput signal to the amplifier from the cathode ray transmitter tubevaries inversely as frequency. A single stage may be designed to providegain which varies directly as frequency over a similar range, however,if two stages having similar frequency response characteristics wereused the gain-would vary as the square of frequency which would notcorrectly compensate for the falling-off characteristic of thecathode-ray transmitter tube.

Two amplifier stages in which each provides gain varying in accordancewith the square root of frequency would-be satisfactory but no simplecircuit is now available to provide such a characteristic.

According to this invention, two or more equalizing circuits havedifferent rising frequency characteristics and are connected in cascadeso that one provides gain substantially proportional to frequency usefulfrequency band and the other or others provide substantially uniformgain for that first part of the useful gain proportional to thefrequency for other parts of the useful frequency band. The responsecharacteristic of the several circuits are also complementary at thetransition points between diiferent parts of the frequency band.

The principalobject of this invention is to provide a. television systemhaving a uniform response over a large range'of frequencies.

vvAnother object of this invention is to provide an amplifier which willcompensate for identical stages in cascade canfor a first part of thefrequency band but provides stray capacity of a transmitter tube and itsassociated circuits. 1

. A still further object of this invention is to provide a multi-stagewide-band amplifier whose frequency response is substantially flat overthe lower portion of the band and increases proportional to frequencyover the higher portion of the band.

Other and incidental objects of this invention will be apparent to thoseskilled in the art from the following specification consi ered inconnection with the accompanying dra s in which t Figure 1 is a circuitof this invention,

Figure 2 is a graph illustrating the operation of this invention, and

Figures 3, 4, 5, 6, 7 and 8 are modifications of this invention. 1

Referring now in more detail to Fig. 1, the amplifier circuit comprisesa cathode ray transmitter tube i having-an evacuated envelope 3 andelectron gun therein for producing a beam of electrons, a second anode land a mosaic I of electron-emissive elements. An optical image of thescene ii to be transmitted is projected upon the mosaic 9 by means of asuitable lens system i 3. -An electrical representation of the opticalimage is formed on the mosaic so that when the mosaic is scanned by theelectron beam from gun 5 it produces picture signals representative ofthe optical image ii. The beam is caused to scan the mosaic 9 by meansof deflecting coils i 5 or any-other suitable deflecting means.

A further and more complete description of the cathode ray transmittertube may be found in an article by V. K. Zworykin appearing in theProceedings of the Institute of Radio Engineers for January 1934, pages16 to 32.

The train of picture signals is transmitted to diagram showing one formthe control electrode ll of the amplifier tube i0 having a cathode andan anode 25; The grid leal: resistor R1 maintains the control electrodei! at a suitable bias potential. The distributed capacity of the picturetube is indicated by the broken line shown by capacity C1. The auxiliaryelectrode is supplied with positive potential through resistance 29 andbypass condenser ii. The anode 28 is supplied with positive potentialthrough resistor 33 and the amplified signal is impressed upon thecontrol electrode ill of the next amplifier tube 31 through couplingcondenser 39. Suitable bias potential is applied to control electrode 30through resistor 4i. Tube 31 contains cathode t3, auxiliary electrodeand anode M. The positive potential for auxiliary electrode 45 issupplied through resistor er 5 I.

A frequency compensating circuit composed of resistances Ra, R: andinductance L1 is connected to the anode circuit of discharge device 31.The modified signal from anode 41 is impressed upon the controlelectrode 59 of tube 8i through coupling condenser 68. Capacity 0:represents the interelectrode capacity of the tubes 37, Si and theirassociated circuits. A second frequency circuit of the anode iii of tube6|. The signal from the anode i3 is then amplified in tube 18 bytransmitting it to control electrode 'ii through coupling condenser 79.The interelectrode capacity of tube 15 is shown by capacity C2.

The anode circuit of tube 3! and the anode 2i, an auxiliary electrode20,

49 and bypass condenswhere fo=maximum frequency utilized (for example,

3,000,000 C. P. S.) f1=a lower transition frequency (for example,

30.000 C. P. S.) and fa=a much higher transition frequency (for example,300,000 C. P. 8.).

Referring now to Fig. 2, T0, T3 and T4, respectively, show the responsecharacteristic curve of the three anode circuits of the three dischargedevices 3 (picture transmitter tube), 37 and ti.

If the falling oil in frequency response of the transmitter tube was tobe compensated for in one stage of amplification, the anode circuit ofthe compensating stage would contain an inductance whose value isgoverned by the following equation:

Li=R2R1C1 (5) where Lr=the inductance in the anode circuit,

Ra=the series resistance in the anode circuit,

R1=theload resistance of the transmitter tube,

and

Ci=distributed capacity of the picture transmitter tube and associatedcircuit in order for the output voltage of the amplifier stage to beuniformly proportional to the current of the picture tube. If thefallingofi in frequency-response characteristic of the picture tube isto be compensated for in a single stage, the valve R: will necessarilybe so low that the irregularities in the power supply voltage will beappreciable by comparison with the picture signal.

It is apparent that Equations 1 and 2 define L1 to have the same valueas was given in Equation 5 for the one amplifier stage frequency response characteristic equalizer. F1 defines the frequency at which thevoltage across L1 has become equal to that. across R: and the voltageacross Lr exceeds the voltage across R: for all higher frequencies.Hence F1 may be called the coming-in" transition frequency for Li. R:has

compensating circuit comprising the resistances R4, R5 and inductanceleis connected to the.

little effect at this frequency because the reactance of L1 isrelatively low with respect to Rs. If R: and C: were not present, thiscircuit would provide correct equalization. At F2 the goin out"transition frequency of L1, the value of Xm reaches R: so that L1 and Rsare equally responsible for the amplification of the stage comprisingtube It.

The voltage across R2 is almost negligible at frequency F2. R; and R:are given such a value that C: will not appreciably load the circuitatthe maximum frequency of the useful band. At the same transitionfrequency F2, the voltage across In in the plate circuit of dischargedevice M becomes equal to the voltage across R4. Above this frequencythe voltage across L2 continues to rise and assumes the taskofcompensating for the continued fall of high frequency response of thepicture transmitter tube i.

For the purpose of this illustration,

curve To tobe substantially fiat m L: by a resistor equal to Xmat Fo,making F atransition frequency. The third network would be addedelsewhere as in anode circuit of tube 15. It would consist of a resistorin se es with an inductance having a reactance equa to the resistor atfrequency F0. The maximum compensated frequency would then become equalto F0.

More equalizing stages could be added by either extending the band or byspacing the transition frequencies closer in the band. It must beobserved that the coming-in transition frequency of each stage is thesame as the, goingon transition frequency of the stage adjacent on thelow frequency side in the frequency band.

It will be noticed that according to one form of this inventionregardless of how many stages of'compensation there may be, theinductance shunting resistor will be omitted in the compensating stagehaving the highest transition frequency. It is, however, satisfactory'touse the in uctance-shunting resistor in each stage providing thegoing-out transition frequency of the stage, having the highest comingin transition frequency, is at the upper limit of the desired band.

Referring now to Fig. tion frequencies F1, F2 and F0 are not as much asseveral octaves apart in frequency, the resistor in series with theinductance becomes compar- 3, if the various transi' sistance circuit aninductance such as L3 in Fig. 6; therefore. the elements Ra, La and C:in efiect combine to produce a resistive load across the channel, thusresulting in a circuit, the equivalent of which is shown in Fig. 3.

Fig. 7 shows still another embodiment of this invention in which acompensated circuit comprising condensers'and resistances is used.Thecapacities and resistances are of such a value that the correspondingstages produce response curves similar to curves is and t4 shown in Fig.2. One

compensating circuit comprises resistance R2 in series with resistanceR6 and series circuit comprising resistanceRa and capacitor C4 connectedin parallel with resistance R2. Another compensating stage includingtube 6|, which has the highest coming-in transition frequency of all thecompensating stages, is provided with a resistance R4 in series withresistance R1 in its anode circuit, resistance R4 having a condenser Csshunted therewith.

A better understanding of the operation of Fig. 7 will'be had withreference to Fig. 2. F1 defines the frequency at which the admittance ofC4 has become equal to that of R2, and the admittance of C4 is more thanthat of R2 for all higher frequencies. R: has little effect at thisfrequency I because the reactance of C4 is relatively high with respectto R3. Hence Fr may be called the coming-in" transition frequency forC4. At F2,

the going-out transition frequency of C4, the i value of X04, (thereactance of C4) reaches R3,

' so that the admittances of C4 and of R3 are qually able with theresistor shunting the inductance.

If this obtains, it is important whether the resistor R3 shunts theinductance L1 alone or shunts the series resistor R2 also, as shown inFig. 3.

The coming-in" transition frequency of 'Li, forexample, then is effectedby R: as well as by R2. Similarly, the goingeout transition frequency isaffected by Rz also instead of by Rs alone, as

indicated by Equations 3 and 4. A slight revision- This circuit iscommonly used for a coupling impedance between the stages for a uniformresponse video signal. amplifier.

It was shown in Fig. 3 that, according to one form of this invention,both the series resistance R2 and inductance L1 are shunted by aresistnegative feedback in ance R3. However, no account was taken of theunavoidable stray capacitance. Fig. 5 specifically takes into accountthe unavoidable stray capacity C2 of the circuit and tube electrodes. Weknow that, in order to provide a circuit which presents a resistive loadto a definite range of frequencies and in which there is included acapacity such as C2, it is necessary to include in the shuntreresponsible for the amplification oi the stage comprising tube 31. Atfrequencies above F2, C4 has little effect because X04 is relativelylowv with respect to R3.

R2 and Re are given such a value that C3 will not appreciably load thecircuit at the maximum frequency of the useful band. At the sametransition frequency Fz, the admittance of C5 in the anode circuit ofdischarge device Bl becomes equal to the admittance of R4. Above thisfre-v quency, the current through C5 continues to increase, thusassuming the task of compensating for the continued fall ofhighfrequency response of the picture'transmitter tube.

Referring now to Fig. 8, there is shown another embodiment of thisinvention in which the compensating circuit of each of the amplificationstages is provided for in the cathode circuit of the discharge devices.Discharge device 31 has in its cathode circuit a series resistance R2shunted by a series circuit comprising capacity C4 and resistance Ra. Atleast one of the stages,

that is, the stage compensating for that highest part of the usefulfrequency range, is provided with a cathode resistor R4 and a shuntcondenser Cs.

By using resistance in the cathode circuit of a discharge the circiut'to control the response characteristics of a tube. By including in thecathode circuit a circuit whose impedance changes with frequency it ispossible to change the frequency response of an amplifier. By referringagain toFig. 2, the operation of the circuit shown inFig. 8 may be morereadily understood. F1 defines the frequency at which the admittance ofC4 has become equal to that of R2, and admittance of C4 is less than theadmittance of R2 for all higher frequencies. Hence, F1 is called thecoming-m transition frequency for the amplifying stage comprisingdischarge device 31 and its associated circuit. R: has little effectdevice, it is possible to introduce enough at this frequency because thereactance of C4 is relatively high with respect to Ra. At F2, thegoing-ou transition frequency of the stage comprising tube 31, the valueof X04 reaches Ra. so that C4 and Rs are equally responsible for theamplification of the stage comprising tube 37. At frequencies above F2,C4 has little effect because x04 is relatively low with respect to Ra.

At the same transition frequency F2, the voltage across C5 in thecathode circuit of the discharge device il becomes equal to the voltageacross R4. Above this frequency, the admittance of C5 continues toincrease and assumes the task of. compensating for the continued fall ofhigh frequency response of the picture transmitter I l V While severalsystems for carrying this invention into effect have been indicated andde-' scribed, it will be apparent to one skilled in the art that thisinvention is by no means to be limited by the particular organizationshown and described but that many modifications thereof will be madewithout departing from the scope of this invention as set forth in theappended claims. 7

I claim as my invention:

i. In combination with a cathode ray pickup tube for generating picturesignals occupying a 'wide frequency band, said tube having high in temalimpedance and having output terminals that have unavoidable capacitytherebetween, an

output resistor connected between said terminals, an amplifier includinga first vacuum tube having input electrodes connected across saidresistor, said resistor having such high resistance that the frequencyresponse characteristic at said input electrodes falls with increaseinfrequency, said amplifier comprising a plurality of amplifier stagesconnected in cascade, one of said stages including a frequencycorrecting network which gives to said one stage a frequency responsechar-' acteristic which has a rising response with increase in frequencyover a band of intermediate frequencies in said wide band and which hasa substantially flat response over the rest of said wide band, andanother of said stages includin a frequency correcting network whichgives to said other stage a frequency response characteristic which hasa rising response with increase in frequency over a band of frequenciesat the high frequency end of said wide band and which has asubstantially flat response over the rest of said wide band, the overallfrequency response characteristic of said two stages with frequenccorrecting networks being substantially complementary to the frequencyresponse characteristic at said input electrodes.

2. In combination with a cathode ray pickup wide band, and another ofsaid stages including 1 tube for generating picture signals occupying awide frequency band, said tube having high internal impedance and havingoutput terminals that have unavoidable capacity therebetween, an outputresistor connected between said terminals, an amplifier including afirst vacuum tube having input electrodes connected across saidresistor, said resistor having such high resistance that the frequencyresponse characteristic at said input electrodes falls with increase infrequency, said amplifier comprising a plurality of amplifier stagesconnected in cascade, one of said stages including a frequencycorrecting network which gives to said one stage a frequency responsecharacteristic which has a rising response with increase in frequencyover a band of intermediate frequencies in said wide band and which hasa substantially flat response over the rest of said a frequencycorrecting network which gives to said other stage a frequency-responsecharacteristic which has a rising response With Incrcase in frequencyover a band of'frequencies toward the high frequency end of said wideband and which has a substantially flat response over the rest of saidwide band, the rising frequency response characteristic of each of saidstages with frequency correcting networks being substantiallycomplementary to the frequency response characteristic at said inputelectrodes for corresponding bands of frequencies.

3; A frequency compensating television signal transmission circuithaving a frequency response characteristic which rises in response tofrequency throughout its entire pass band and comprising in combinationa plurality of amplifier stages each having a frequency responsecharacteristic which does not vary with frequency except wherein theresponse characteristic rises in response to frequency over a portion ofsaid band other than that portion of said band in which each of theother of said stages have rising frequency characteristics, and whereinsaid portions make up the entire band.

4. A frequency response compensating television signal transmissioncircuit having a frequency response characteristic which risessubstantially proportional to frequency over a rela tively widefrequency range comprising in combination a plurality of signalamplifying stages each of which has a frequency response characteristicwhich is substantially flat except wherein the response characteristicrises substantially proportional to frequency over a different portionof said frequency range and wherein said portions make up substantiallythe entire frequency range.

'ALDA V. BEDFORD.

